1. Field of the invention
This invention relates to a photosensitive resin composition and a photosensitive resin composition for an i-line stepper which contains a photoinitiator.
2. Prior art
In the semiconductor industries, inorganic materials have conventionally been used for interlaminar insulation. Recently, organic materials having an excellent heat-resistance such as a polyimide resin have been put to use as materials for interlaminar insulation because of properties thereof.
A process for forming a pattern in a semiconductor integrated circuit or forming a pattern of a circuit on a print substrate comprises complex and various steps such as film formation of a resist material on a substrate surface; exposure of required portion to light; removal of unnecessary portion by etching or the like; and washing of the substrate surface. Therefore, development of heat-resistant photosensitive materials have been desired, which enables the required portion of the resist material to be remained as such and used as insulating materials even after the pattern is formed by exposure to light and development.
As such a material, heat-resistant photosensitive materials comprising, for example, a photosensitive polyimide or a cyclized polybutadiene as a base polymer have been proposed. The photosensitive polyimide has particularly attracted attentions since it has an excellent heat-resistance and impurities contained therein can easily be removed.
As such a photosensitive polyimide, for example, one which comprises a polyimide precursor and a bichromate has been proposed for the first time in Japanese Patent Publication No. 17374/1974. This photosensitive polyimide has an advantage that it has a photosensitivity suitable for practical uses and also has a high film-forming ability. However, it also has a disadvantage that it has a low preservability and a low stability and that a chromium ion remains in the polyimide, and therefore it has not been put to practical use.
As another example, a photosensitive polyimide precursor in which a photosensitive group is introduced into a polyamic acid (polyimide precursor) by an ester bond has been proposed in Japanese Patent Publication No. 30207/1980. This material has a disadvantage that a finally obtained product contains a chloride since a step for introducing the photosensitive group comprises a dehydrochlorination reaction.
In order to avoid these problems, for example, a process for mixing a compound which contains a photosensitive group with a polyimide precursor is disclosed in Japanese Provisional Patent Publication No. 109828/1979; and a process for affording photosensitivity to a polyimide precursor by reacting a functional group in the polyimide precursor with a functional group of a photosensitive group-containing compound is disclosed in Japanese Provisional Patent Publications No. 24343/1981 and No. 100143/1985.
However, the photosensitive polyimide precursor employs an aromatic monomer having an excellent heat-resistance and mechanical property as a fundamental structure and has a low light-transmittance in the ultraviolet region because the polyimide precursor itself absorbs the ultraviolet light. Therefore, photochemical reactions at the exposed portion are not sufficiently caused which results in the low sensitivity or unclear patterns.
Recently, the higher and higher reduction has increasingly been required for a rule for producing a semiconductor, accompanied by the higher integration of semiconductors. Therefore, in addition to a conventional contact/proximity exposing machine using parallel rays, a 1:1 projection exposing machine called as a mirror projection and a reduced projection exposing machine called as a stepper have increasingly been used. The stepper utilizes monochromatic light such as a high power oscillation line of ultra-high pressure mercury lamp, an excimer laser. As the stepper, a g-line stepper which employs a visible light (wavelength of 435 nm) called as a g-line of ultra-high pressure mercury lamp has conventionally been used in many cases. However, further reduction of processing rule has been required. The process has already been carried out around the lower limit of diffraction of light and therefore it is required to shorten the wavelength of the stepper used for carrying out finer processing. Thus an i-line stepper having a wavelength of 365 nm has increasingly been used instead of the g-line stepper having a wavelength of 435 nm. However, a base polymer of a conventional photosensitive polyimide designed for the contact/proximity exposing machine, the mirror projection exposing machine or the g-line stepper, having a low transparency for the above-described reason, has substantially no transmittance particularly for the i-line having a wavelength of 365 nm. Therefore, the i-line stepper does not provide any useful pattern. On the other hand, as a polyimide film for surface protection, a further thicker film has been required in response to a LOC (lead on chip) which is a high density assembly method of a semiconductor element. When such a thicker film is used, the low light-transmittance causes more serious problem. For the above reasons, a photosensitive polyimide which is designed for the i-line stepper and has a high transmittance for the i-line has been highly required.